1. Field of the Invention
The present invention concerns a casing for a mobile telephone usable in a public network or in a private installation. More generally, the invention concerns all casings that reproduce a sound message and that may need to be pressed against the ear.
2. Description of the Prior Art
Existing mobile telephones incorporate a loudspeaker to emit sound opposite a perforated area of a casing. The user places this area, and therefore the loudspeaker, near or against his or her ear in order to hear messages. To make it possible to use a casing of this type in "hands free" mode, it is possible to switch the mobile telephone to a mode in which the loudspeaker emits the message with sufficient power to be heard all around. These two modes of use have mutually contradictory constraints.
If the user has the casing pressed against his or her ear, for personal listening, low-frequency components of the message transmitted to the user are overamplified. Thus a standard requires the amplitude within an audible frequency band to be contained within specified limits. This makes it necessary to be able to filter the low-frequency components to reduce their amplitude. The electronic filtering usually employed requires the interpolation of a filter for processing the signal before it is converted into an audible message.
In hands free listening, on the other hand, low-frequency components are necessary for effective diffusion of the signal into a large space, for example an office. Accordingly, on changing from one mode of use to the other it is necessary to switch low-frequency filters into or out of service.
In practice, the cost of the filters adds to the price of the device.
Creating acoustic leakage in the perforated area of the casing to establish communication between the open air and a cavity formed between the loudspeaker and an inside face of the casing against which the loudspeaker is pressed, in order to damp low frequencies, is known per se. With a solution of this kind, compliance with the limits of the standard unfortunately cannot be obtained without adding a complementary electronic filter.